A submarine cable communication system bears large-capacity communication services between continents. Failures of submarine optical cables and devices have a huge impact on communication. An objective that the submarine cable system pursues constantly is to locate a failure point timely and precisely and accurately reflect a failure mode. A failure locating method based on an optical time-domain reflectometry (OTDR, Optical Time-Domain Reflectometry) technology gradually becomes a mainstream failure locating method of the submarine cable system. An optical repeater of the submarine cable communication system usually includes an optical isolator, and a backscattered loopback signal after the isolator cannot be returned. Therefore, in the submarine cable system, a loopback channel of an OTDR backscattered loopback signal is established in a device of an optical isolation area, thereby implementing line detecting of the repeater submarine cable system.
In a line detecting solution using the OTDR technology, usually an output-to-output loopback manner or an output-to-input loopback manner is used.
FIG. 1 is a schematic diagram of an output-to-output backscattered signal loopback detecting principle. A submarine cable line terminal device and a submarine cable line detecting device are arranged on the land, and a submarine cable line including an optical repeater and an optical cable is placed in the seafloor. The submarine cable line detecting device sends a detection signal to the submarine cable line through a coupler, where the detection signal passes through the submarine cable line and is returned to the submarine cable line detecting device through the coupler. The loopback manner in the optical repeater establishes a common backscattered loopback channel at the output ends of a pair of amplifiers in opposite directions. The downlink line of the loopback channel is from the output end of the downlink amplifier to the output end of the uplink amplifier; the uplink line of the loopback channel is from the output end of the uplink amplifier to the output end of the downlink amplifier. There are coupling apparatuses at the output ends of the downlink amplifier and uplink amplifier. In FIG. 1, when a breakpoint exists in the optical cable, the breakpoint may be known according to power of the backscattered loopback signal received by the submarine cable line detecting device.
The power of the backscattered loopback signal received by the submarine cable line detecting device has the following relationship:
                              P                      RX            ⁢                                                  ⁢            _            ⁢                                                  ⁢            N                          =                              P            TX                    +                                    ∑              1              N                        ⁢                          G              i              down                                -                                    ∑              1                              N                -                1                                      ⁢                          IL              i              down                                +                      B            s                    -                      IL            N            loopback                    +                                    ∑              1                              N                -                1                                      ⁢                          G              i              up                                -                                    ∑              1                              N                -                1                                      ⁢                          IL              i              up                                                          (                  Formula          ⁢                                          ⁢          1                )            
PRX—N indicates power of a detection signal which is returned from the Nth span;
PTX indicates a detection signal generated by the line detecting device;
Gidown and Giup respectively indicate gains of the downlink amplifier and uplink amplifier of the ith optical repeater;
ILidown and ILiup respectively indicate losses of fibers in the downlink path and uplink path in the ith span;
Bs indicates a ratio of power of a backscattered loopback signal, is related to the fiber, and usually is a fixed value; and
ILNloopback indicates a loopback loss in the Nth optical repeater.
If a submarine device is normal, an obtained detection curve is shown by the solid line in FIG. 2. In FIG. 2, the horizontal axis represents time, and the vertical axis represents power of the backscattered loopback signal. The power of the backscattered loopback signal changes with the time, as shown by the solid line in FIG. 2. Each span represents the change of the power of an optical repeater and an optical cable connected to the optical repeater.
To improve reliability, in two amplifiers in an optical repeater of a submarine cable, usually a pump redundancy manner is used. To be specific, forward and reverse optical amplifiers share a pair of pump lasers to provide energy. When one of the pump lasers fails, output power of the two amplifiers is reduced by 3 dB, and at the same time, gains are also reduced by 3 dB. This means that input power of the next optical repeater is also reduced by 3 dB. Because all optical repeaters in the line work in a deep saturated status, when the input power of the next repeater is reduced by 3 dB, a gain of the repeater is automatically increased by 3 dB. As may be known by analyzing the above features of the gain, when a pump fails, for example, a pump in the second optical repeater fails, the change of the detection curve is shown by the dotted line in FIG. 2, which shows that the detection curves of the second and third spans are lowered slightly. The detection curve cannot reflect changes of the gain and output power of the optical repeater in the line, and it is difficult to identify a pump failure event.
In FIG. 3, the upper part includes three spans, and each span includes a first half-span optical cable and a second half-span optical cable. The span between optical repeaters is large. With the loopback manner, the status of only a part of the optical cable within the span can be detected, for example, the first half-span optical cable, while the detection curve of the second half span is covered by noise. The detection curve is shown by the wave part in the upper part of FIG. 3. In this case, if a breakpoint occurs in the second half-span optical cable or the next optical repeater fails, the detection curve is shown by the lower part of FIG. 3. The wave part in the lower part of FIG. 3 is similar to the wave part in the upper part of FIG. 3, and therefore, it is difficult to distinguish whether the cable is broken or the next optical repeater fails.
FIG. 4 is a schematic diagram of an output-to-input backscattered signal loopback detecting principle. In the loopback manner, two separate backscattered loopback channels are respectively established at the output ends and input ends of a pair of downlink and uplink amplifiers in opposite directions and in the optical repeater, which are used for detection in different directions. There are coupling apparatuses at the input end and output end of the downlink amplifier and the input end and output end of the uplink amplifier. The downlink line of the loopback channel is from the output end of the downlink amplifier to the input end of the uplink amplifier; the uplink line of the loopback channel is from the output end of the uplink amplifier to the input end of the downlink amplifier.
Power of a backscattered loopback signal received by the submarine cable line detecting device has the following relationship:
                              P                      RX            ⁢                                                  ⁢            _            ⁢                                                  ⁢            N                          =                              P            TX                    +                                    ∑              1              N                        ⁢                          G              i              down                                -                                    ∑              1                              N                -                1                                      ⁢                          IL              i              down                                +                      B            s                    -                      IL            N            loopback                    +                                    ∑              1                              N                -                1                                      ⁢                          G              i              up                                -                                    ∑              1                              N                -                1                                      ⁢                          IL              i              up                                                          (                  Formula          ⁢                                          ⁢          2                )            
PRX—N indicates power of a detection signal which is returned from the Nth span;
PTX indicates a detection signal generated by the line detecting device;
Gidown and Giup respectively indicate gains of the downlink amplifier and uplink amplifier of the ith optical repeater;
ILidown and ILiup respectively indicate losses of fibers in the downlink path and uplink path in the ith span;
Bs indicates a ratio of power of a backscattered loopback signal, is related to the fiber, and usually is a fixed value; and
ILNloopback indicates a loopback loss in the Nth optical repeater.
If a submarine device is normal, an obtained detection curve is shown by the solid line in FIG. 5. As shown by the dotted line in FIG. 5, when a pump in an optical repeater fails, for example, a pump in the second optical repeater from left to right in FIG. 4 fails, the detected power change is shown by the dotted line, which shows that the detection curve of the first span rises by 3 dB and that the detection curve of the second span is lowered by 3 dB. The detection curve cannot truly reflect changes of the gain and output power of each optical repeater in the line, and the pump failure event always needs to be analyzed and confirmed through the uplink and downlink bidirectional detection curves.
In an application scenario of long spans, a detection curve is shown in the upper part of FIG. 6. With the loopback manner, the status of only a part of an optical cable within a span can be detected, for example, the first half-span optical cable, while the detection curve of the second half span is covered by noise. The noise part is shown by the wave part in the upper part of FIG. 6. In this case, if a breakpoint occurs in the second half-span optical cable or the next optical repeater fails, the detection curve is shown in the lower part of FIG. 6, and the power detected in the second span is increased. The wave part in the lower part of FIG. 3 is similar to the wave part in the upper part of FIG. 3, but it is still difficult to distinguish the two failure modes.
The detection results of the above two manners show that: detecting the gain of light passing through all optical repeaters and the sum of losses of all optical cables can neither detect the gain of the amplifying unit of an optical repeater in one direction, nor detect validity of the pump of an optical repeater. Likewise, for a submarine optical functional module, for example, an optical equalizer or an optical splitter, failures cannot be located accurately, and as a result, a performance detecting and failure locating cannot be performed for the submarine device accurately.
At present, a dynamic range of the commercial coherent OTDR meter in the submarine cable field is about 17 dB, and the corresponding span line-of-sight distance of the optical cable is about 85 km. With the development of submarine cable transmission technologies, the span distance of the submarine cable in some areas is increased from 70 km to more than 100 km and even to 120-150 km, and the coherent OTDR detection capability already cannot satisfy the requirement for accurately locating a long-span system failure.